The invention relates generally to apparatus and packaging for optical switching systems and more particularly to packaging for an array of optical switching mirrors having reduced internal wiring connections which is accomplished by incorporating a ball grid array to connect first and second printed circuit boards associated with the array of mirrors.
In recent years optical fibers have come into wide spread use in a wide variety of applications in which optical signals are transmitted along such fibers and are switched from one fiber to another by means of an optical switch. Conventional optical switches generally include structure to support fiber positioning, alignment signal emitters and interconnected computer control electronics. A fiber positioning structure is provided near the end of each fiber to selectively point the end of a fiber in one fiber group toward the end of a selected fiber in another fiber group to provide switched optical transmission between the two fibers. An alignment signal emitter is provided near an end of and in predetermined spaced relationship to the end of each fiber to emit an alignment signal for controlling the fiber positioning structure when aligning the ends of selected fibers in the fiber groups for switched optical transmission there between. Examples are shown in U.S. Pat. Nos. 4,512,036 and 5,177,348. This approach requires considerable complexity and duplication of alignment structure and circuits for each alignable fiber. It would be advantageous to reduce this complexity and duplication and to increase speed of switching, reliability, as well as to reduce cost in implementation.
Texas Instruments presently manufactures a two-axis analog mirror MEMS device fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 115 m. The layout consists of an oval mirror (normally about 3.8 mmxc3x973.2 mm) supported on a gimbal frame by two silicon torsional hinges. The gimbal frame is attached to a support frame by another set of torsional hinges.
When provided as an individual mirror switching device, the number of electrical connections for input and output signals is small enough that they can reasonably be managed by conventional wire or terminal connections. However, an array of mirrors (such as a 10xc3x9710 array) results in such a large number of input/output connections (on the order of about 500 to 600 connections), it is extremely difficult to use conventional wire connections and terminals.
Although not limited to such use, the present invention is particularly suitable as a drive apparatus for the two-axes analog mirror optical switch manufactured by Texas Instruments of Dallas, Tex., or according to another embodiment, an array of such optical switches that overcomes the limitations of the prior art, and which is relatively low in cost, has high speed and is reliable in operation.
For example, presently available optical transmission switches available from Texas Instruments employ a microelectromechanical movable mirror assembly with associated electromagnet coils and may also include control LED""s with both drive and LED signals supplied through a standard connector or wiring harness. The drive signals to the electromagnetic coils, and to and from the positional electronics presently requires a nine or ten wire connector and/or harness. However, it is believed that the number of input/output signals per two-axes mirror may be reduced to about five or six signals.
The mirror is typically mounted to a support structure of suitable material, such as ceramic, along with the magnetic coil driving means and a wiring harness. The package is received in a housing in which an optical fiber is received and in which another mirror is disposed in alignment with the fiber for reflecting an optical signal from the fiber to the movable mirror.
Objects and advantages of the invention will in part be obvious, and will in part appear hereinafter, and will be accomplished by the present invention which provides drive apparatus having reduced internal wiring connections. The drive apparatus is well suited for providing necessary forces such as electromagnetic forces or electrostatic forces for rotating a mirror about at least one axes. The drive apparatus comprises a base printed circuit board which may be made of a ceramic or any other suitable material having a top side and a connecting side. There is a plurality of connecting points on the topside of the base printed circuit board. There is also included at least one drive module such as for example an electromagnetic coil or alternately an electrostatic plate located on the topside of the base printed circuit board and which has input connections. A second or support printed circuit board having a device side and a backside covers the drive module and includes a plurality of connecting points on the backside which corresponds to the plurality of connecting points on the topside of the base printed circuit board. Typically, the plurality of connecting points on the topside of the base printed circuit board is a mirror image of the plurality of connecting points on the backside of the support printed circuit board. There is also included a plurality of conductive balls mounted on one of either the base printed circuit board connecting points or the support printed circuit board connecting points such that selected ones of the connecting points of the support printed circuit board are in electrical contact with selected ones of the plurality of connecting points on the base printed circuit board. Thus, it will be appreciated that connections are made between the backside of the support printed circuit board and the topside of the base printed circuit board by sandwiching the conductive balls there between. It should also be understood that the conductive balls may be mounted to either of the base printed circuit board or the support printed circuit board. Alternately, a portion of the conductive balls could be mounted to the base printed circuit board and the remainder mounted to the support printed circuit board. As mentioned above, the drive apparatus typically is used for providing rotational forces to an optical switching mirror mounted above the drive apparatus. Further, the rotating force may be an electrostatic force or an electromagnetic force provided by at least one coil. According to one embodiment, a single coil may be used to cause rotation of the switching mirror about an axis by providing current flow in one direction. Likewise, rotation can be caused in the opposite direction by reversing the current flow. The rotation around the axis may also be accomplished by using two drive coils rather than a single coil. Likewise, as is discussed in detail hereinafter, rotation of the mirror about two axes may be accomplished by using two coils or four coils.
Thus, it is seen that the mirror of the present invention may provide either one or two axes of rotation and may be driven magnetically or alternately may be driven by electrostatic forces. The mirror used in the assembly is preferably made from a single piece of crystalline material such as silicon and has three portions connected by two sets of hinges. An inner portion forms the mirror. One of the hinge pairs, one hinge on each of two opposite sides of the mirror portion, ties the mirror portion and the middle gimbals portion, which surrounds the mirror portion. This allows the mirror portion to rotate about the gimbals portion, providing the first axis of rotation. The second set of hinges ties the gimbals portion and the frame portion, one hinge on each of two opposite sides on a line disposed, preferably orthogonal or 90xc2x0 relative to a line drawn through the first set of hinges. This allows the gimbals portion, which carries the mirror, to rotate about the frame portion, providing a second axis of rotation.
In one embodiment, two pair of magnets, one for each axis of rotation, are used to move the mirror portion and are mounted to form a mirror assembly. The first pair of magnets are attached by suitable means to the mirror portion of the mirror assembly, one on each of two opposite sides of a line, 90xc2x0 relative to a line through the mirror/gimbals portions set of hinges. When subjected to a selected magnetic field, the mirror portion rotates about the mirror/gimbals portions set of hinges, providing the first axis of motion. The second pair of magnets are suitably attached to the gimbals portion of the mirror assembly, one on each of two opposite sides of a line, 90xc2x0 relative to a line drawn through the gimbals/frame portions set of hinges. In the same manner as discussed above, when subjected to a different magnetic field, the mirror and gimbals portions rotate about the second set of axis, to providing the second axis of rotation.
To obtain extended operation without degradation, the mirror assembly may be hermetically assembled into a cavity in the package to lock out moisture and allow the provision of a benign atmosphere for mirror operation. The cavity can be filled with selected gases to provide improved heat transfer and, if desired, exclude oxygen water vapor and other materials that would adversely affect the mirror over time.
According to one embodiment, the coil drive module preferably employs a push and pull arrangement for driving the mirror magnets to rotate the mirror portion to the desired orientation in its two axes. Four coils, comprising copper wire coiled on a bobbin may be included in the module. The coil leads from the modules are soldered to the ball grid array printed circuit board (BGA-PCB) to allow system electrical control of the coils and their push pull arrangement to drive the mirror assembly. The coil bobbins are preferably made of aluminum or other eddy current generating material, and sufficient amounts of aluminum should be provided at the top and bottom of the bobbins to allow eddy current dampening of the movable portions of the mirror assembly, to prevent unwanted oscillations. In order to prevent overheating and loss of mirror position control, the coil bobbins are made of high heat transfer material, such as aluminum, and the bobbins are massive relative to the coils.